In recent years, there has been devised a technique referred to as Multimedia Broadcast Multicast Service (MBMS) that distributes multi contents such as animation or music over an existing network (such as W-CDMA based 3G mobile phone network). The MBMS utilizes an existing network to broadcast multi contents from a base station to the terminal devices (or multicasts multi contents to a plurality of designated terminal devices).
When the base station broadcasts (or multicasts) multi contents for transmitting to terminal devices, it is required that a header (Internet Protocol <IP>/User Datagram Protocol <UDP>/Real-time Transport Protocol<RTP> header or the like) containing various control information is added to the multicast contents to generate packets. However, since the amount of data of the header occupying the packet is large and the amount of data of the multi contents storable in the packet is limited, the multi contents have not necessarily been distributed efficiently.
On the one hand, various header compressing techniques for compressing a header include Robust Header Compression (ROHC), for example (see Japanese translation No. 2007-502073 of PCT international application). When using the ROHC, it is required that one packet transmission source is for one transmission destination and a compression state between the transmission source and the transmission destination is fed back.
FIG. 15 is a diagram for explaining a base station 10 that transmits packets based on the ROHC and a terminal device 20 that receives the packets. As illustrated, the base station 10 synchronizes the compression state with the terminal device 20 to transit to any of the IR (initialize) state, the First Order (FO) state and the Second Order (SO) state, and generates packets corresponding to the transited state.
There will be explained a packet that is generated by the base station 10 at each state. FIG. 16 is a diagram of one example of data structure of the packet transmitted at each state. FIG. 16A is a diagram of a data structure of an IR packet generated by the base station 10 at the IR state. FIG. 16B is a diagram of a data structure of an FO packet (IR-DYN packet) generated by the base station 10 at the FO state. FIG. 16C is a diagram of a data structure of a SO packet (UO-0/UO-1/UO-2) generated by the base station 10 at the SO state.
As depicted in FIG. 16A, when the base station 10 is at the IR state, the header compression is not performed and the uncompressed header (IP/UDP/RTP header) is stored in the IR packet. The IR packet has image data (multi contents) stored therein in addition to the header.
As depicted in FIG. 16B, when the base station 10 is at the FO state, minimal header compression is performed and the minimally-compressed header (ROHC header) is stored in the FO packet. The header has image data (multi contents) stored therein in addition to the header. The ROHC header contains Dynamic Part (Chain) corresponding to a time stamp of the RTP header.
As depicted in FIG. 16C, when the base station 10 is at the SO state, maximal header compression is performed and the maximally-compressed header (ROHC header) is stored in the SO packet (the amount of data of the header in the SO packet is smaller than that of the header in the FO packet).
On the other hand, the terminal device 20 synchronizes the compression state with the base station 10 to transit to any of the No Context state, the Static Context state and the Full Context state to receive packets from the base station 10. Specifically, when the base station 10 is at the IR state, the terminal device 20 transits to the No Context state and receives the IR packet.
When the base station 10 is at the FO state, the terminal device 20 transits to the Static Context state (or the Full Context state) to receive the FO packet. While the terminal device 20 is receiving the FO packet at the Full Context state, if an elongation error or the like occurs, the terminal device 20 transits to the Static Context state. When the base station 10 is at the SO state, the terminal device 20 transits to the Full Context state to receive the SO packet.
When the header contained in the packet is compressed (when the terminal device 20 has received the FO packet or SO packet from the base station 10), the terminal device 20 elongates compressed header's information and reproduces multi contents by the ROCH technique.
FIG. 17 is a diagram for explaining a state transition sequence of the ROHC. As illustrated, the base station 10 (compressor) transits to the IR state, the FO state and the SO state over time, and transmits the packet corresponding to each state to the terminal device 20 (decompressor). Further, the terminal device 20 also transits its state depending to the state of the base station 10.
The ROHC has not been applicable to the MBMS in the conventional art, and there has been a problem that throughput for multi contents distribution could not be improved.
Specifically, when the ROHC is applied to the MBMS, a plurality of headers need to be transmitted depending on the state of each terminal device. However, since the transmission radio area of the MBMS has only one area (channel) on downlink, data having several types of header cannot be transmitted at the same time.
When the ROHC is applied to the MBMS, the compression state needs to be synchronized between the base station and the terminal devices. However, since the relationship between the base station and the terminal devices is assumed to be 1:n (n is more than 1), the base station could not synchronize the compression state with each terminal device.